Why are the mesh faces different colors?
While I can't tell you why they're in this state currently, as there are many possible reasons, these colors indicated the direction in which the face normals point. To fix this, you need to recalculate your faces' normals, by pressing Ctrl + N. Alternatively, this can be done from the toolbar (whose visibility is toggled by pressing T),by pressing Recalculate found in the Shading/UVs tab of said toolbar.